Sizing a propane hot water heater means matching its First-Hour Rating to your household’s busiest 60 minutes of hot-water use, not to the nominal tank size.
Pick a tank by gallons and you can still run cold mid-shower, because a 50-gallon heater with weak recovery may deliver fewer usable gallons in an hour than a 40-gallon unit with a strong burner. Propane changes the math: gas-fired storage heaters recover faster than electric models at the same tank size, so the number that decides whether you run out is the First-Hour Rating. Here is how to calculate your peak-hour demand, match it to a unit, and check the code requirements before installation.
What Is a First-Hour Rating And Why Does It Beat Tank Size?
First-Hour Rating (FHR) is the amount of hot water a storage heater can supply in the first hour, starting with a fully heated tank, per the Department of Energy. It is the figure to size against because it combines two things: the hot water already in the tank plus what the burner can reheat during that hour. A bigger tank holds more stored water; a stronger burner recovers more. FHR is the only published spec capturing both, which is why the International Code Council notes that tank capacity alone does not tell you how much hot water you can use in your busiest hour.
For propane, that recovery half is the advantage: gas-fired storage heaters generally recover faster than electric units of the same tank size, so a modest propane tank can post an FHR that beats a larger electric one.
One scope note: FHR is a storage-heater metric. Tankless propane units are sized by flow rate and temperature rise, so this method does not apply to them.
How Do You Calculate Your Peak-Hour Demand?
Add up every hot-water draw that realistically overlaps during your busiest 60 minutes. CenterPoint Energy’s sizing guidance is exactly this: identify the hour of greatest use, total the hot-water amounts in that hour, and choose a heater whose first-hour rating is close to that total.
Walk through the hour your household actually clusters — usually morning or evening — and count what runs at the same time:
- Showers — the biggest draw, and the one most likely to stack back-to-back.
- Bathroom and kitchen faucets — handwashing, shaving, rinsing.
- Dishwasher — one full cycle in that hour.
- Clothes washer — a warm or hot wash running while someone showers.
Say two showers and a dishwasher land in the same hour. That total is your peak-hour demand. Compare it to each candidate unit’s published FHR, not its tank size, and pick one at or slightly above your figure. Sizing to the busiest hour — not average daily use and not headcount alone — keeps the last shower from going cold.
| Peak-Hour Situation | What It Demands | Sizing Move |
|---|---|---|
| One shower, nothing else overlapping | Low, single-fixture draw | Smallest FHR that clears it |
| Two showers back-to-back | High, sustained draw | FHR at or above the combined total |
| Shower plus dishwasher | Two fixtures at once | Total both, then match FHR |
| Shower plus laundry | Two high-volume draws | Total both, then match FHR |
| Morning cluster in a full house | Multiple overlapping draws | Size to the full hour, not the average |
| Big tank, weak burner | Stored water only | Check FHR — gallons can mislead |
| Smaller tank, strong propane burner | Storage plus fast recovery | FHR can beat a larger electric unit |
If your peak-hour total lands in the low range — one or two fixtures, nothing stacked — compact tanks can cover it, and a tested roundup of 10 gallon propane hot water heater models worth comparing is a useful starting point for a small household.
What Mistakes Skew Propane Sizing Most Often?
- Sizing by tank gallons. Gallons describe storage, not usable hot water in an hour. FHR answers the real question, per the ICC.
- Sizing by household size alone. Four people who shower at different times need less than three who all shower at 7 a.m. Peak-hour timing, not headcount, drives the number.
- Ignoring simultaneous demand. A shower plus a dishwasher is two draws at once, and both belong in the total.
- Forgetting recovery. On propane, recovery is the strength — but only if you count it. FHR already includes it; tank gallons do not.
Before purchasing, check local requirements. The ICC advises verifying your jurisdiction’s adopted plumbing code, since code and permitting vary by location. Propane installation must also match the site’s gas supply, venting, and clearances — model- and jurisdiction-specific requirements the installer confirms.
FAQs
Does a bigger propane tank always mean more hot water?
No. A larger tank stores more hot water, but FHR also counts what the burner can reheat in that hour. A smaller propane tank with a strong burner can deliver a higher FHR than a larger unit with weak recovery, so compare ratings rather than gallons.
Can I use this method to size a propane tankless unit?
No. FHR applies to storage heaters. Tankless propane units are sized on flow rate and temperature rise, so a tankless purchase needs a different calculation entirely. Use the peak-hour method only for storage tank models.
How do I know if my current heater is undersized?
The clearest sign is running out of hot water during your busiest hour — usually the second or third shower, or when the dishwasher runs alongside one. Compare your peak-hour total to the unit’s published FHR; if demand exceeds it, the heater is the bottleneck.
References & Sources
- CenterPoint Energy. “Residential Water Heater Sizing Guide.” Supports identifying the hour of greatest use and matching a heater’s first-hour rating to that total.
- International Code Council. “Water Heater Sizing and Location: Code vs. Practicality.” Supports why tank capacity alone is insufficient and the need to check adopted local plumbing codes.
- U.S. Department of Energy. First Hour Rating definition, DOE test procedure docket. Defines First-Hour Rating as hot water supplied in the first hour from a fully heated tank.
